Motion capture is an approach to generating motion data that is based on tracking and recording the movement of real objects. One common application of motion capture is in animation where a realistic sequence of motion, e.g., by a human actor, can be captured and used to represent the motion of an animated object.
In some motion capture systems, an actor wears a black body suit. A number of white balls are attached to the suit at the actor's joints, e.g., shoulder, elbow, wrist. The actor then performs a sequence of movements which is digitally recorded by a number of cameras. The recorded data is then processed by a motion capture program.
The motion capture program recognizes the white balls as points. If multiple cameras record the same point in a frame and the locations of the cameras are known, the motion capture program can determine the 3D position of the point using triangulation. The motion capture system may determine 3D positions for all of the points in each of the frames. As used herein, the term “frame” refers to a period of time, or a collection of different periods of time, at which a 3D position of an object being captured is calculated.
The 3D points may be input into a fitting program that fits a virtual skeleton, which represents the bone structure of the object being captured by the system, to the 3D points. For example, an upper arm bone can be defined to exist between a point associated with a shoulder and a point associated with the elbow. The motion capture program may track the movement of the points from frame to frame, which, once fitted to the virtual skeleton, defines the movement of the skeleton from frame to frame.
In some systems, however, use of triangulation may require a relatively large number of cameras to capture each of the differently positioned points from multiple camera angles. Although only two cameras may be required to triangulate a single point, the two cameras typically need to track the point during each frame. As a result, if only two cameras are used, the cameras can lose track of a white ball as a result of the actor turning around. Thus, when an actor performs a scene with lots of motion, a large number of cameras, e.g., 20, are typically utilized to ensure that at least two cameras can track each white ball during each frame.
In some motion capture systems, it may be difficult to determine which white ball recorded with a first camera at a first angle corresponds with the same white ball taken from a second camera at a second angle. When the white balls recorded by two cameras are not correctly associated, the fitting program may produce some artifacts in the form of physically impossible motions in an attempt to fit the skeleton to the 3D points. For example, a leg can suddenly appear to move 180° from one frame to the next frame. While dramatic changes can be easy to spot, subtle mistakes may be difficult to catch.
Additionally, some motion capture systems tend to require refined lighting conditions. For example, the white balls can be formed as objects that reflect infrared (IR) light, and the cameras may be tuned to respond to IR light. When an IR light source is directed toward the white balls, the cameras pick up the points of IR light reflected back from the actor. This approach, however, may not work well on a traditional movie set because when the lights are turned up on a movie set they become quite hot and unintentionally flood the scene with IR light. As a result, the cameras may not be able to distinguish a point of reflected IR light from background noise.